1. Field of the Invention
The invention relates to anthraquinone dyes and somewhat more particularly to pleochromatic anthraquinone dyes, their synthesis and utilization.
2. Prior Art
Intensive work has been under way for a considerable period of time to develop a so-called "absorptive" liquid crystal display in which a pleochromatic dye is provided as an additive in a liquid crystal layer (see Applied Physics Letters, Vol. 38, page 91, 1968). Such a display is particularly characterized in that it provides a large angular range of observation and operates without polarizers. Despite these attractive properties, liquid crystal displays with dye additives have not yet become completely competitive with more conventional displays primarily due to the fact that no dye has yet been developed which not only dissolves in a sufficient amount in available liquid crystal materials and is stable over prolonged storage and operating time periods which also provides acceptable contrast.
Primarily, a display contrast depends on the degree to which the liquid crystal molecules can transfer their respective orientation state to an embedded dye molecule. Characteristics for the degree of ordering of a dye molecule is the magnitude S=1/23&lt;cos.sup.2 &gt;-1), which is attained by means of averaging over the angle between the individual dye molecules and the director of the liquid crystal grid.
In an ideal situation, when all dye molecules lie parallel to the preferred direction of the liquid crystal molecules, S assumes the value 1 and, in practice lies clearly below such value. (Calculation feasibilities for S as well as the interrelationships between the ordering parameters and the contrast perceived by an observer are set forth in IEEE Trans. on Electron Devices, Vol. ED-24, page 823, 1977).
Specific azo dyes which, for example, are described in Electronics Letters, Vol. 12, page 514, 1976, have a relatively high degree of ordering. However, it has been noted that it is precisely those azo compounds which have the highest S values (with S=0.79) and the greatest coefficients of absorption, decompose even after a few 1000 hours and quickly fade under UV loads.
Anthraquinone-based dyes are relatively stable and were therefore early investigated for their suitability as additives in liquid crystal layers. Thus, German Auslegeschrift No. 24 45 164 describes 1,4-alkylphenylamino anthraquinones as well as 1,4- and 1,8-alkylamino anthraquinones but, from the data presented in this prior art publication, these anthraquinones only effect a weak image contrast. If, as suggested in German Offenlegungsschrift No. 28 15 335, the anthraquinone ring is substituted at its 1 and 4 position or, respectively, 1 and 8 positions, with the radical --N.dbd.CH--C.sub.6 H.sub.4 --X (wherein C.sub.6 H.sub.4 is a phenyl group and X is selected from the group consisting of a nitro-, cyano-, phenyl-, C.sub.1-20 -alkyl-, C.sub.1-20 -alkoxy- or a di-C.sub.1-4 -alkylamino group), a relatively high degree of ordering is attained. The peak values of 0.9 and above described in German Offenlegungsschrift No. 28 15 335, however, cannot be reproduced with the pertinent liquid crystal/dye mixtures. Further, this class of compounds is available in only a very few colors; with the anthraquinone ring substituted at the 1 and 4 position being blue, and the anthraquinone ring substituted at the 1 and 8 position having a reddish coloration.
A somewhat greater color spectrum is available when, as suggested in European Offenlegungsschrift No. 0 00 21 04, the anthraquinone ring is substituted at its 1, 4 and 5 positions as follows: an anilino-group (substituted or modified) is at the 1 position; either hydrogen or a hydroxyl group is positioned at the 4 position; and the 5-position radical is, in a first instance, an anilino group (substituted or modified) and, in a second instance, a hydrogen radical. Chemically, particularly photochemically, this class of dyes is extraordinarily resistant to degradation but does not orientate particularly well; further, as a rule, the S-values for these groups of compounds remain below 0.7 and at best achieve an S-value of 0.71.